There is no law that states an existing system "has to be" converted to R 134 a if a system using R-12 is functioning properly.

If it needs recharging due to a leak,and you have a source for the R -12 then consider getting it fixed.

Many technicians will try to sell you retrofit kits,but if your system is low on charge,it may only need an o ring or tightening of a fitting. Have a reputable garage leak check the system and find out an action plan for a repair.

R -12 systems in good working order will cool down much faster than 134a systems.

Beware though, that the cost of any remaining R-12 refrigerant on earth is going to be very expensive, that's why many car guys retrofit to the newer R-134a systems. Hope that helps
HR 20